My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Sunday, 21 December 2008

Sticking point

sid, who is a regular contributor to the RepRap forums, had an idea to get a soldering iron manufacturer to make a heater barrel assembly for RepRap. He approached a Chinese company with a specification and they sent him some prototypes. He forwarded one to me for testing. It appears that they ignored his specification and just sent a standard de-soldering iron element. Nevertheless it is a nice unit and looks eminently usable.

It has a tube running through the middle with an internal diameter a shade over 3mm. Ideally it needs to be about 3.5mm to cope with the worst filament I have encountered. My green ABS, being a little undersized, fits down it easily.

The heater has a cold resistance of 1.3Ω but, unlike nichrome, it has a big temperature coefficient, so its resistance increases significantly at it gets hot. It appears that it is a 12V 50W heater. We can drive this with PWM using a MOSFET provided the PSU can handle 9A peaks on the 12V rail in addition to what the steppers take, a tall ask. An inductor and diode could be used to reduce the peak current.

The other two wires are a type-E thermocouple. Unfortunately the thermocouple sensor board that Zach designed using the AD595 is for the more common type-K thermocouples. It can be recalibrated for type-E by adding extra resistors. However, the AD595 is an expensive chip because it is factory trimmed for accuracy. By the time you add external components the convenience and accuracy is lost so you might as well just use a cheap op-amp and a micro with an internal temperature sensor for the ice point compensation. E.g. the MSP430F2012 that I use for my extruder controller is a lot cheaper solution than the AD595 and can control the heater and motor as well.

To test the heater I clamped it by the mounting flange in a vice and hooked it up to a bench power supply. I measured the internal thermocouple's output with a millivoltmeter and also inserted a 3mm rod type-K thermcouple down the barrel. Here are the results: -

Voltage

Current

Power

Resistance

Temperature

Thermocouple

Calculated Temp

1 V

0.75 A

0.8 W

1.3 R

43 C

1.5 mV

42 C

2 V

1.20 A

2.4 W

1.7 R

106 C

5.6 mV

102 C

3 V

1.55 A

4.7 W

1.9 R

182 C

9.5 mV

160 C

4 V

1.80 A

7.2 W

2.2 R

275 C

14.5 mV

233 C

5 V

2.00 A

10.0 W

2.5 R

357 C

20.0 mV

314 C

The temperature column is as measured with my type-K thermocouple towards the nozzle end of the barrel. The calculated temp column is assuming 68μV/°C from the type-E thermocouple and a cold junction temperature of 20°C. There is a big temperature gradient along the barrel so the thermocouple reading depends on where it is placed.

As you can see we only need about 5V to drive the heater. The current would start at 3.8A and fall to 2A as it warmed up. This would be kinder to the PSU and safer than using 12V, but 12V would give a much faster warm up time. I expect something better than bang-bang control would then be needed to avoid massive overshoot.

When running horizontally the inlet tube stays cold and the mounting flange is just too hot to hold so it would be ideal for mounting to ABS or HDPE. This is because the barrel appears to be stainless steel, which is a very poor conductor of heat. The element must be towards the bottom so there is a continuous thermal gradient along the barrel.

The nozzle that came with it is made from copper with some type of plating. It had a hole to mate with the tube that sticks out of the end of the heater but it did not go all the way through. In fact it could not, as the tip comes to a fine point. I suspect this is a soldering iron bit that has been drilled out to fit.

I attempted to drill a 0.5mm hole through it but it just snapped the drill. Even drilling a 1mm hole snapped the drill. In the end I drilled a 2mm hole, but the drill bent and came out the side. I think it needed to be sharper for copper. Finally, I cut the point off and filled the 2mm hole with high temperature solder. That is soft enough to easily drill a 0.5mm hole through. It melts at 300°C so should hold up.

The heat damage is where I heated it up with a blow torch in an attempt to remove the broken drill bits. Copper expands a lot more than steel. That did not work so I tried to get it red hot to soften the drill bits so I could drill them away. I failed to get it red hot but I did melt the plating.

The shape is not ideal for making objects but it is good enough to see if I can extrude. In fact it extrudes well. I was able to push a piece of ABS through it easily by hand and it extruded at a very good rate.

The bit/nozzle is clamped on to the end of the barrel by an outer stainless steel sleeve tightened up by a threaded ring at the cold end. I was worried it would leak under extrusion pressure without some sealing. When I stripped it down I found it did leak a little but didn't get far. I suspect it freezes when it meets the outer sleeve.

So apart from the bore being a little too small this seems like a perfect solution: -

It needs no construction apart from drilling the nozzle.

It is mechanically sturdy.

It should be very durable; soldering irons last a lifetime and they run at higher temperatures.

It is cold enough to mount with plastic without any insulation. It does after all in a soldering iron although that is probably a thermoset plastic.

The nozzle can be easily removed and replaced.

BUT, it has one fatal flaw, exactly the same flaw as my attempt to use stainless steel as a heat barrier in my experimental high temperature extruder. There is a slow thermal gradient along the length of the barrel. That means there will be a point about half way along where it is just hot enough to soften the plastic but not hot enough to make it flow. When you push a piece of virgin filament in it slides past that point easily because it does not have time to soften until it gets to a much hotter part of the tube. If however you stop extruding then the stationary filament has enough time to soften further up. When you push it again it expands to plug the tube but is not fluid enough to move. No matter how much force you apply you cannot move it. To get it going again you have to pull it out backwards, cut off the swollen bit and start again.

The reason the original extruder design does not have this problem is that the thermal gradient is in the PTFE. It is much shorter so the problem region that is soft but not molten is a lot shorter and the walls are very slippery so it can still be shifted.

I can't think of a solution to this problem. You could make the internal tube out of copper but then the top end would be hot so you would need a PTFE thermal break again. Also it would not be an off the shelf product, it would be custom to RepRap. Perhaps a taper at the problem region could stop it sticking.

The next extruder I am building has an aluminium barrel and nozzle and a PEEK thermal break. It won't suffer from this problem at least.

4 comments:

Right after I got into RR work, I reviewed most of the patents available from stratasys.

They of course solved the thermal gradient problem as well. They did it by doing two things:

(1) they use smaller diameter filament, and

(2) they use cooling air flow to allow cool filament to enter a hot nozzle. In the strat machines, it appears that there is about 1/2" of stainless steel thin-wall tubing exposed to cooling air, and the thermal gradient from 270C to ambient is accomlished in that small length. The strat machines use a thin walled tube that is bolted into an alum. block.

I do not know to what extent the diameter of the filament contributes to the fact that their design works. In my extruder, I'm currently still using PTFE, but designed it with the extruder barrel assembly not mechanically connected via the PTFE so that later I could experiment with cooling air instead of the PTFE piece.

At a minimum i've managed to remove the PTFE from being a structural member, which seems to help.

Interesting, one of things I have been meaning to try was to simply shorten the stainless steel pipe in my high temp design. Looks like it would work but wastes a fair bit of energy. I was only losing about 1.4W with 50mm so if I reduced it to 12mm I would lose 5.8W which is about 50% of the heater power required. Still not much compared to the total power to run the machine.

I am making a PEEK insulator at the moment as the MetaLab guys report good results with it. It is a lot stronger than PTFE. Quite nice to machine.